The present invention relates to a method and an apparatus for the measurement of a resistance of a switching contact of an electrical power switch (a so-called circuit breaker) as well as a method and an apparatus for the measurement of resistances of switching contacts arranged in a series connection of an electrical circuit breaker. In particular, the present invention relates to a measurement of transition resistances or contact resistances of a closed switching contact or of closed switching contacts of such an electrical circuit breaker.
Power switches, which are also called high voltage switches, load switches or circuit breakers, are used in the field of power engineering or energy technology to establish an electrical connection under load or to disconnect such a connection. The nominal voltages of circuit breakers may be in the range from a few volts to some hundred kilovolts. in the event of a short circuit, the switched load currents may be in the range of some ten kiloamperes. Therefore, for a reliable operation of the circuit breaker, for example the transition resistance of a switching contact or of a plurality of switching contacts connected in series of the electrical circuit breaker are tested in the course of revisions.
Circuit breakers for medium voltage equipment usually have only one switching contact which can be opened or closed. Circuit breakers in high voltage or ultra-high voltage systems can comprise a plurality of switching contacts, so-called interrupter units, in a series connection. In such a series connection of a plurality of interrupter units, generally capacitors dimensioned in the range of some pikofarads are arranged in parallel to the individual interrupter units so as to uniformly distribute the voltage over the individual interrupter units. In general, several interrupter units in a phase of a circuit breaker are opened and closed at the same time.
The resistance measurement at a closed switching contact, which is also referred to as micro-ohm measurement, is for circuit breakers a standard process for the assessment of the quality or the wear condition of the circuit breaker.
The micro-ohm measurement is usually conducted by impressing a high direct current of 100 ampere, for example, over the closed switching contact. For this purpose, the current is supplied via current clamps that are clamped at both sides of the circuit breaker to the conductors that lead away from the circuit breaker. With the aid of further clamps the voltage is tapped at both sides of the circuit breaker as well. The voltage clamps are usually mounted closer to the switching contact of the circuit breaker, a 4-wire measurement being conducted with this arrangement. This prevents that the voltage drop at the current clamps is measured with the measurement, which would falsify the measurement result. The resistance of the closed switching contact, including the resistance of the conductors from the voltage clamps to the switching contact, can be determined from the impressed current and the measured voltage. Alternatively, so-called Kelvin clamps may be used instead of separate current and voltage clamps. In Kelvin clamps, two jaws of a respective clamp are electrically isolated from one another, and the current is supplied via one of the two jaws, while the voltage is tapped via the other of the two jaws. The advantage of such Kelvin claws resides in that only one clamp is to be clamped at each side of the circuit breaker.
As described above, for the micro-ohm measurement a current source and a voltage meter can be used so that voltage measurements can be successively conducted at the different switching contacts. A plurality of voltage meters may be used as well, the current being impressed with the aid of a common current source via several contacts and a plurality of voltage values being determined with the plurality of voltage meters.
As dangerous high voltage can occur at many places in a power engineering installation, for example in a transformer station, it is necessary that the circuit breaker is grounded during the micro-ohm measurement. For example, the circuit breaker can be disconnected at both sides from the remaining energy network and can be grounded at one side. The micro-ohm measurement can then precisely be conducted when the switching contact is closed or when the switching contacts are closed. Often further measurements have to be conducted at the circuit breaker, which require that the switching contact is at least temporarily opened, for example a measurement of the time that takes the switch to open. For such measurements a grounding at both sides of the switch is recommendable to avoid that persons conducting the measurement are exposed to danger. Therefore, for the micro-ohm measurement, one of the two groundings will be removed for the duration of the measurement, which however is very cumbersome, or in the case of a grounding at both sides the micro-ohm measurement becomes inaccurate due to the parallel grounding loop.
In order to be able to efficiently conduct a micro-ohm measurement at a circuit breaker, the circuit breaker may be grounded at both sides, and with a DC-current clamp or a shunt the portion of the current which flows from the current source through the grounding equipment can be determined and can be used to correct the measured resistance. While this method is very precise, it is disadvantageous in that additional measurements are necessary by means of the current clamp or the shunt.
Therefore, it is the object of the present invention to enable an efficient resistance measurement or micro-ohm measurement for one or a plurality of switching contacts of an electrical circuit breaker with exposure of personnel conducting the resistance measurement to danger being avoided.